Here Is What You Need To Know About Tropical Depression Eighteen

Tropical Wave 49 in the far Eastern Atlantic has developed into Tropical Depression Eighteen at 5:00 PM Wednesday 22nd September 2021. This depression is forecast to slowly organize as it moves westward and takes a northwest path as a major hurricane near the Lesser Antilles. It is still too soon to tell if this tropical cyclone could impact the Lesser Antilles. Interests in the Leeward Islands should monitor this system closely.

What we know

Tropical Depression Eighteen's Forecast Track as of 5:00 AM Thursday 23rd September 2021. (National Hurricane Center)
Tropical Depression Eighteen’s Forecast Track as of 5:00 AM Thursday 23rd September 2021. (National Hurricane Center)

Tropical Wave 49 (or Invest 98L) gradually became better organized on Wednesday afternoon, prompting the National Hurricane Center to issue advisories. Based on satellite-derived wind data, there is still a broad low-level but barely closed circulation, but peak winds between 28 and 30 knots were detected, sufficient to determine it was a tropical cyclone.

Based on the latest information from the National Hurricane Center, the center of Tropical Depression Eighteen was located near 10.5°N, 36.4°W, approximately 2,720 kilometers east of Trinidad and Tobago. The depression is moving toward the west near 24 km/h, and this general motion is expected to continue through Friday. A slower motion toward the west-northwest is expected Friday night and continuing into the weekend.

Tropical Depression Eighteen in the Main Development Region of the Atlantic Ocean. (Weathernerds)
Tropical Depression Eighteen in the Main Development Region of the Atlantic Ocean. (Weathernerds)

The satellite presentation of Tropical Depression Eighteen shows the system maintaining strength, with curved bands to the north and west of the center. Still, there is some dry air wrapping into the circulation from the east.

Presently, Tropical Depression Eighteen is located in an area with warm sea surface temperatures between 28°C and 29°C, low wind shear, and favorable upper-level winds.

This system is forecast to become a tropical storm later today. The next name on the list of names for the 2021 Atlantic Hurricane Season is Sam. When this depression is named, it will join 2020 as the only other hurricane season where 18 storms formed by 23rd September.

Where is Tropical Depression Eighteen Going?

The takeaway: Everyone in the Lesser Antilles should monitor this system, but those in the Leewards should pay particularly close attention to the eventual forecast track. This system is approximately 6-7 days away from its closest approach to the Leewards. This system poses no direct threat to the Windward Islands, including Trinidad and Tobago at this time.

Tropical Depression Eighteen’s forecast tracks from top dynamical and consensus hurricane models as of 2:00 AM Thursday 23rd September 2021. (NCAR)
Tropical Depression Eighteen’s forecast tracks from top dynamical and consensus hurricane models as of 2:00 AM Thursday 23rd September 2021. (NCAR)

This tropical depression is riding along an expansive low to a mid-level high-pressure ridge to the north, which is why the system will continue to mostly westward at relatively low latitude for the next 4-5 days (by Monday, latest).

Models continue to show anomalously strong 500-mb ridging developing over the eastern and central Caribbean
Sea in 2 to 3 days, which is likely to block the depression’s forward progress towards the start of next week. This means the eventual tropical storm or hurricane will slowly move towards the Lesser Antilles through the weekend and through the first half of next week.

However, as this system strengthens, it is forecast to gradually gain latitude, taking the Southern Windward Islands out of the eventual tropical storm and hurricane’s crosshairs. By Sunday into Monday, this tropical cyclone will be approaching the western extent of the high-pressure ridge, which will also be retreating eastward. This steering pattern should take the hurricane northwestward, potentially missing the Lesser Antilles.

European (ECENS) versus the American (GEFS) latest ensemble model runs showing the stark differences in the track of Tropical Depression Eighteen approximately 7 days out (by Thursday 30th September 2021) (Weathernerds)
European (ECENS) versus the American (GEFS) latest ensemble model runs showing the stark differences in the track of Tropical Depression Eighteen approximately seven days out (by Thursday 30th September 2021) (Weathernerds)

Looking at the two top global models at a longer range, the American GFS takes the tropical cyclone well northeast of the Leewards. In contrast, the European EMCWF takes the system uncomfortably close to the northernmost Leewards. Nearly all long-range models show the eventual storm moving either across or northeast of the Leewards by next Wednesday into Thursday.

Effectively, with the slow movement and the interaction with the multiple high-pressure ridges, this means it will be a few more days before models, and the National Hurricane Center, have a better idea of the eventual path of this system as it nears or potentially moves across parts of the Lesser Antilles.

How Strong Can Tropical Depression Eighteen Get?

The takeaway: By the end of the upcoming weekend, a slow-moving hurricane is becoming increasingly likely east of the Lesser Antilles, and it is too soon to tell where it may be impacted.

Tropical Depression Eighteen's forecast intensity guidance from top dynamical, statistical, and consensus hurricane models as of 2:00 AM Thursday 23rd September 2021. (NCAR)
Tropical Depression Eighteen’s forecast intensity guidance from top dynamical, statistical, and consensus hurricane models as of 2:00 AM Thursday 23rd September 2021. (NCAR)

Environmental conditions are favorable for the storm to gain strength over the next several days. Based on the current forecast, the cyclone is forecast to move over warm sea surface temperatures between 28°C and 29°C in an environment with low wind shear, less than 10 knots, and relatively high moisture.

Most intensity models show steady strengthening over the next five days, including the National Hurricane Center’s official forecast.

The National Hurricane Center is explicitly forecasting Tropical Depression Eighteen to strengthen into a tropical storm later today when it will be called Sam. Then, by Saturday 25th September 2021, Sam is forecast to become a hurricane, and by Tuesday 28th September 2021, a major Category 3 hurricane is forecast.

Tropical Depression Eighteen probabilities for hurricane-force winds as of 5:00 AM Thursday 22nd September 2021. (National Hurricane Center)
Tropical Depression Eighteen probabilities for hurricane-force winds as of 5:00 AM Thursday 22nd September 2021. (National Hurricane Center)

However, top global models (GFS, EMCWF, and the UKMET) all show the cyclone’s surface circulation does not tighten up for another 24 to 48 hours which may slow intensification.

A stronger tropical cyclone will gain more latitude, while a weaker one takes a more westward path, potentially putting some Lesser Antilles Islands at risk.

What impacts can we expect to see from Tropical Depression Eighteen?

This system is 6-7 days away from the closest approach to the Lesser Antilles, and there is still considerable uncertainty. Continue to check the latest information from your respective meteorological offices and the National Hurricane Center.

Tropical Depression Eighteen probabilities for tropical-storm-force winds as of 5:00 AM Thursday 22nd September 2021. (National Hurricane Center)
Tropical Depression Eighteen probabilities for tropical-storm-force winds as of 5:00 AM Thursday 22nd September 2021. (National Hurricane Center)

This system is still several days away, so it is too soon to tell where, if any, may see tropical-storm-force or hurricane-force winds and rainbands directly associated with the eventual Tropical Storm/Hurricane Sam.

Regardless of its path, based on the forecast strength, long-period swells are likely to begin affecting coastlines of the Lesser Antilles from Monday 27th September 2021, which can produce life-threatening surf and rip currents.

As this system moves to the northwest, potentially taking the cyclone northeast of the Leewards, a trough extending from the cyclone’s core to the southwest will slacken the pressure gradient, causing light winds. Light winds, high moisture, and a somewhat favorable atmospheric setup will allow for high temperatures triggering localized, slow-moving afternoon showers and thunderstorms across western and hilly areas. In addition, funnel clouds and waterspouts will also be possible.

Specifically for Trinidad and Tobago, in addition to the localized activity outlined above and the long-period swells, winds from the south will also be possible. This wind profile will result in showers and thunderstorms moving from the south to the north, possibly intensifying over the Gulf of Paria and the Northern Range and producing hazardous seas as well as street/flash flooding along the East-West Corridor.

But this model shows…

Individual model runs are just one possible outcome from a myriad of outcomes. Weather does not always follow what is modeled, and even what may be forecast. Beware of individual model runs being posted on social media.

Always check the National Hurricane Center for the latest information for tropical cyclones and your local meteorological offices for country-specific advisories.

What should I do?

This system is not forecast to affect the Southern Windward Islands, including Trinidad and Tobago. Don’t panic. There are no immediate or direct tropical storm threats heading to T&T, so feel free to ignore the “storm coming” chain messages that pass through WhatsApp every hurricane season.

If you are a risk-averse person, now is a good time to check your inclement weather, flood, or hurricane season plan, ensuring your preparedness supplies are not expired, stocked, and in a safe location.

The Office of Disaster Preparedness and Management has compiled a comprehensive guide for preparing for the Wet and Hurricane Season.

Tropical Cyclone Climatology

2021 has already produced 17 tropical cyclones in the Atlantic Basin, with the following system being named Sam for the 2021 Atlantic Hurricane Season.

As we head through the second half of September and into October, we’ll continue to monitor the entire Atlantic closely as tropical cyclones could form from tropical waves, non-tropical low-pressure systems in the North Atlantic, and from the Central America Gyre in the Western Caribbean Sea or the southwestern Gulf of Mexico.

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